This invention relates to yellow light-emitting halophosphate phosphors. In particular, this invention relates to hallow phosphate phosphors activated with Eu2+ and Mn2+ that emit yellow light upon being excited by near ultraviolet (xe2x80x9cnear UVxe2x80x9d)-to-blue electromagnetic radiation. This invention also relates to light sources incorporating such halophosphate phosphors to generate white light.
A phosphor is a luminescent material that absorbs radiation energy in a portion of the electromagnetic spectrum and emits energy in another portion of the electromagnetic spectrum. Phosphors of one important class are crystalline inorganic compounds of very high chemical purity and of controlled composition to which small quantities of other elements (called xe2x80x9cactivatorsxe2x80x9d) have been added to convert them into efficient fluorescent materials. With the right combination of activators and host inorganic compounds, the color of the emission can be controlled. Most useful and well-known phosphors emit radiation in the visible portion of the electromagnetic spectrum in response to excitation by electromagnetic radiation outside the visible range. Well-known phosphors have been used in mercury vapor discharge lamps to convert the ultraviolet (xe2x80x9cUVxe2x80x9d) radiation emitted by the excited mercury vapor to visible light. Other phosphors are capable of emitting visible light upon being excited by electrons (used in cathode ray tubes) or x rays (for example, scintillators in x-ray detection systems).
The efficiency of a lighting device that uses a phosphor increases as the difference between the wavelength of the exciting radiation and that of the emitted radiation narrows. Therefore, in the quest for improving efficiency of white light sources, effort has been dedicated to finding a source of stimulating radiation that has wavelengths longer than that of UV radiation and phosphors that respond to those wavelengths. Recent advances in light-emitting diode (xe2x80x9cLEDxe2x80x9d) technology have brought efficient LEDs emitting in the near UV-to-blue range. The term xe2x80x9cLEDsxe2x80x9d as used herein also includes laser diodes. The term xe2x80x9cnear UVxe2x80x9d as used herein means UV radiation having wavelengths in the range from about 315 nm to about 410 nm. These LEDs emitting radiation in the near UV-to-blue range will be hereinafter called xe2x80x9cUV/blue LEDs.xe2x80x9d As used herein, a UV/blue LED may emit radiation having wavelengths in the near UV range, in the blue light range, or in a broad range from near UV to blue. It would be an advance to the technology of lighting to provide a range of phosphors that can be stimulated by the radiation emitted from these UV/blue LEDs radiation sources to allow for a flexibility in the use of phosphors for generating various color LEDs. Such phosphors when combined with the emission from the UV/blue LEDs can provide efficient and long lasting lighting devices that consume little power.
Many near UV/blue LEDs based on combinations of nitrides of indium, aluminum, and gallium have recently appeared. For example, U.S. Pat. No. 5,777,350 discloses LEDs comprising multiple layers of InGa and p- and n-type AlGaN, which emit in the wavelength range from about 380 nm to about 420 nm. A LED of the InGaN type emitting blue light wavelengths was combined with a coating of a yellow light-emitting yttrium aluminum garnet phosphor activated with cerium (xe2x80x9cYAG:Cexe2x80x9d) to produce white light and is disclosed in U.S. Pat. No. 5,998,925. Similarly, U.S. Pat. No. 6,066,861 discloses an yttrium aluminum garnet phosphor activated with terbium and/or cerium in which yttrium may be substituted with Ca and/or Sr, aluminum with Ga and/or Si, and oxygen with S, to be used as a component of a wavelength conversion layer for a blue light-emitting LED. YAG:Ce and its variations emit a broad-spectrum yellow light. Although a substantial portion of the need for white light devices may be filled by LED-based devices, the ability to combine a UV/blue LED with a phosphor has been limited because yttrium aluminum garnet phosphor and minor variations thereof have been the only known yellow light-emitting phosphors that are excitable by radiation in the blue range. This limitation has restricted, to some extent, the ability flexibly to design light sources having different color temperatures and achieving a high color rendering index (xe2x80x9cCRIxe2x80x9d).
Therefore, there is a need to provide phosphor compositions that are excitable in the near UV-to-blue range and emit in the visible range such that they may be used flexibly to design light sources having tunable properties, such as color temperature and CRI.
The present invention provides europium and manganese co-activated halophosphate phosphors that are excitable by electromagnetic radiation having wavelengths in the near UV-to-blue range (from about 315 nm to about 450 nm) to emit efficiently a visible light in a range from about 440 nm to about 770 nm. The emitted light has a broad spectrum with a peak in the range from about 550 nm to about 650 nm and has a yellow-to-orange color. A halophosphate phosphor of the present invention comprises two activators of Eu2+ and Mn2+ and has a general formula of Aa(PO4)3D:Eu2+,Mn2+ wherein A is selected from the group consisting of Ca, Sr, Ba, Mg, and combinations thereof; D is selected from the group consisting of F, Cl, OH, and combinations thereof; and a is in a range from about 4.5 to and including 5.
In one aspect of the present invention, a halophosphate phosphor is combined with at least one other phosphor that has a peak emission in the blue-green wavelength region (from about 450 nm to about 550 nm) to provide a white light. Such other phosphors may be selected from the group of Sr4Al14O25:Eu2+ (herein after called xe2x80x9cSAExe2x80x9d, peak emission at about 490 nm), Sr6P6BO20 (peak emission at about 480 nm), BaAl8O13 (peak emission at about 480 nm), A5(PO4)3Cl:Eu2+ (peak emission at about 480 nm) where A is defined above, and Sr2Si3O6.2SrCl2 (peak emission at about 490 nm).
In still another aspect of the present invention, a europium and manganese co-activated halophosphate of the present invention, either alone or in a mixture with one or more phosphors enumerated above, is disposed adjacent to a near UV/blue LED to provide a white-light source.
Other aspects, advantages, and salient features of the present invention will become apparent from a perusal of the following detailed description, which, when taken in conjunction with the accompanying figures, discloses embodiments of the present invention.